I guess it depends on which side you want to look at.
What NIB price are you going off of?
That $120 internet ball is going to cost another $40-70 to get drilled - maybe more depending if you use grips/slug and if the shop charges more. I believe my local shop is up to $60 for a blank drill and $15 for grips/slug.
That $120 ball just set you back $195, but people only think it's worth no more than $60. If you're lucky they will add shipping to that, but most likely they will want that to include shipping.
Now, depending on the drill, most people will end up just getting the thumb plugged and redrilled. That's $15-20 around here, and another $10 for a slug.
Now the person that spent $195 on a ball is selling it for 1/3 of what they paid, and the new person is getting a ball for 1/2 the cost of a new ball.